1. Field of the Invention
The present invention relates to a switching regulator circuit for controlling power supplied to a load.
2. Description of the Related Art
Conventionally, a switching regulator circuit is provided between a power supply and a load in order to control power supplied to the load. The switching regulator circuit controls a voltage and a current supplied to the load by repeatedly switching whether or not supply of the power to a coil provided in series with the load is allowed as disclosed in Japanese Patent Application Laid-Open No. 2001-215913, pages 6-12 and FIGS. 2-6, for example.
FIG. 1 shows an exemplary conventional switching regulator circuit 300. The switching regulator circuit 300 is provided between a power supply 200 and a load 250. The switching regulator circuit 300 includes a high-voltage input terminal 306, a low-voltage input terminal 308, a high-voltage output terminal 310, a low-voltage output terminal 312, a switching device 302, a regulating diode 314, a smoothening capacitor 316 and a coil 304.
The high-voltage input terminal 306 and the low-voltage input terminal 308 are connected to a high-voltage terminal and a low-voltage terminal of the power supply 200, respectively. The high-voltage output terminal 310 and the low-voltage output terminal 312 are connected to a high-voltage terminal and a low-voltage terminal of the load 250, respectively.
The coil 304 is provided between the high-voltage input terminal 306 and the high-voltage output terminal 310 in series, and the switching device 302 is provided between the coil 304 and the high-voltage input terminal 306 in series. The switching device 302 supplies a voltage lower than a power-supply voltage to the load 250 by repeatedly going on and off. More specifically, when the switching device 302 has been turned on, a current is supplied from the power supply 200 to the coil 304. Then, when the switching device 302 has been turned off, a current is supplied to the coil 304 via the regulating diode 314. In a case where the switching device 302 has been turned off, the current supplied from the power supply 200 to the coil 304 is reduced to zero. Therefore, for compensating this reduction of the current, the coil 304 receives the current supplied to the load 250 via the regulating diode 314. Thus, a linear current is supplied to the load 250.
Conventionally, a field-effect transistor is used as the switching device 302. Since the switching device 302 is provided between the high-voltage input terminal 306 and the high-voltage output terminal 310 as described above, it should be formed by a p-channel fiend effect transistor. In other words, it is difficult to use an n-channel field effect transistor because it required application of a voltage higher than the power-supply voltage to a gate terminal thereof.
The conventional switching regulator circuit 300, however, has a problem that, in a case of using the switching device 302 having a predetermined breakdown voltage and a predetermined on-resistance, a chip area becomes larger. This is because the conventional switching regulator circuit 300 uses the p-channel field effect transistor as the switching device 302. Moreover, the p-channel field effect transistor has low carrier mobility. Therefore, a high-speed operation of the p-channel field effect transistor is difficult.
In addition, there are a few types of p-channel field effect transistor in the marketplace. Therefore, it is difficult to select a p-channel field effect transistor having desired characteristics.